Organic isocyanates are generally produced on a large commercial scale by the reaction of the corresponding amines with phosgene. Because of the toxicity of phosgene, considerable time has been spent trying to find a method of synthesis of organic isocyanates which would eliminate the need for phosgene. One such method consists of reacting organic nitro compounds with carbon monoxide and organic hydroxyl compounds to the corresponding urethanes and then decomposing these into isocyanates and compounds containing hydroxyl groups. This decomposition reaction can be preceded by a modification of the urethane obtained as intermediate product. Thus, for example, the urethane obtainable from nitrobenzene, carbon monoxide and ethanol could first be reacted with formaldehyde to form the bisurethane of 4,4'-diisocyanatodiphenyl methane, which could then be converted into 4,4'-diisocyanatodiphenyl methane with elimination of the ethanol.
The decomposition of urethanes into the corresponding isocyanates and hydroxyl compounds has been described, for example, in German Offenlegungsschrift No. 2,421,503 and the prior publication cited therein.
Methods for the preparation of urethanes described in the patent literature include the reaction of nitro compounds with carbon monoxide and alcohols in the presence of selenium or selenium compounds (German Offenlegungsschriften No. 2,343,826 (U.S. Pat. No. 3,895,054): 2,614,101 and 2,623,694 (U.S. Pat. No. 4,080,365) or of noble metals, in particular palladium, in the presence of Lewis acids (German Offenlegungsschriften Nos. 1,568,044 and 2,603,574).
For the preparation of a mononitro compound, this reaction proceeds in accordance with the following stoichiometric equation: EQU R--NO.sub.2 +3CO+R'OH.fwdarw.RNHCO.sub.2 R'+2CO.sub.2.
The general reaction equation is as follows: EQU R(NO.sub.2).sub.x +3xCO+xR'OH.fwdarw.R(NHCO.sub.2 R').sub.x +2xCO.sub.2.
This means that for each mol of urethane group to be produced three mols of carbon monoxide are used up and two mols of carbon dioxide are formed. Only one-third of the carbon monoxide put into the process is used for the formation of urethane groups. Two-thirds are converted into the technically useless inert carbon dioxide. Large amounts of heat are evolved in the exothermic formation of carbon dioxide. Therefore, expensive equipment for removing the heat of reaction is necessary in the known industrial method of synthesizing a urethane from a nitro compound, carbon monoxide and alcohol.
The patent literature also teaches how to react N,N'-dialkyl ureas or N,N'-diaryl ureas with alcohols to produce a urethane and an amine, for example, U.S. Pat. No. 2,409,712. The following represents this reaction scheme: EQU RNHCONHR+R'OH.fwdarw.RNHCO.sub.2 R'+RNH.sub.2.
In this method, however, only one-half of the alkyl or aryl groups in the urea is converted into urethane while the other half is converted to the free amine.